To cope with an increase in volume of information concomitant with popularization of the Internet and Local Area Network (LAN), optical interconnection technology that uses optical signals is being under development not only in communication fields of main line and access type but also in short distance signal transmission between boards or in boards of routers and server devices. Specifically, to enable use of light in short distance signal transmission between boards or in boards in routers and server devices, optical/electronic boards that include an electric printed wiring board and an optical transmission path are under development.
In this case, it is desirable to use as the optical transmission path an optical waveguide that has a higher degree of freedom in wiring and is capable of being provided in higher density than optical fibers. Among others, optical waveguides made of polymer materials, which are excellent in processability and cost performance, show promise. Polymer optical waveguides have a structure that is adapted to coexist with electric printed wiring boards as mentioned above, they are required to have high heat resistance in addition to high transparency (low transmission loss). As materials for such an optical waveguide, fluorinated polyimides (see, for example, Patent Document 1 and Non-patent Document 1), deuterized silicone resins (see, for example, Non-patent Document 2), and epoxy resins (see, for example, Patent Document 2 and Non-patent Document 3) have been proposed.
On the other hand, the optical waveguides for use in the above-mentioned utility are required to have a core size of generally 50 μm square to ensure tolerance of connection with a light receiving or emitting device. This means that the core layer must have a thickness of about 50 μm. However, when materials for waveguides including, for example, deuterized silicone resins or fluorinated polyimides are used, there arises a problem in that it is difficult to realize a thickness of about 50 μm on an optical/electronic board or, if it is possible to realize, precision of film thickness will be poor because the materials for waveguides are generally include solvents having low viscosities although the resins themselves have high heat resistance to endure about 300° C.
Further, when fluorinated polyimide waveguide materials, which themselves have high heat resistance to endure about 300° C. and high transparency as high as 0.3 dB/cm at a wavelength of 850 nm, are used, film formation on an electric printed wiring board was difficult to be performed since film formation requires heating at 300° C. or more for several tens minutes to several hours. Further, fluorinated polyimides have no photosensitivity, so the method of fabricating optical waveguides by exposure to light and development can not be applied thereto, and they thus have poor productivity and poor applicability to large-area fabrication. Further, since optical waveguides are fabricated by a film forming method that involves applying a liquid material on a substrate, management of film thickness is cumbersome and in addition, the resin applied on the substrate is still liquid before curing, so the resin will flow on the substrate to make it difficult to maintain uniformity of film thickness. Thus, there are many problems arising from the fact that the form of the material is liquid.
Further, the upper cladding after the core has been embedded must have flatness taking into consideration of subsequent mounting of light receiving or emitting devices. However, when liquid waveguide materials are used for the upper cladding, there tends to occur unevenness as a result of following up the ridge pattern of the core, so it is difficult to realize flatness.
The epoxy resins have problems similar to those of the above-mentioned waveguide materials including deuterized silicone resins or fluorinated polyimides because the epoxy resins are liquid.
That is, heretofore, epoxy resins for forming optical waveguides are those epoxy resins that are liquid at room temperature, or solid epoxy resins diluted with solvents have been used. These epoxy resins have excellent transparency and heat resistance at about 200 to 280° C. However, since an epoxy resin is used for fabricating optical waveguides by applying a liquid material on a substrate and forming a film by, for example, a spin coating method, management of film thickness is cumbersome and in addition, the resin applied on the substrate is still liquid before curing, so the resin will flow on the substrate to make it difficult to maintain uniformity of film thickness. Thus, there are many problems arising from the fact that the form of the material is liquid.
Further, the epoxy resin is capable of forming core patterns by an exposure to light and development method by addition of an optical polymerization initiator and is reported to have a high transparency of 0.1 dB/cm. However, the epoxy resins generally have heat resistance of 200 to 280° C. and to obtain high reliability, they are required to have still higher heat resistance although some of them are applicable to the above-mentioned optical/electronic board.
As mentioned above, none of the conventional resins for forming optical waveguides has in combination (1) high transparency, (2) high heat resistance, (3) high-precision film formability, and (4) acceptable productivity.
Further, in high speed, high-density signal transmission, between electronic devices or printed wiring boards, transmission through the conventional electric wiring is approaching to a limit of attaining high speed and high density due to restrictions of mutual interference and attenuation of signals. To break through such restrictions, the technology of connecting electronic devices and printed wiring boards to each other by means of light, so-called optical interconnection is being studied. As the light path, flexible optical waveguides having flexibility are considered to be suitable from the viewpoints of ease of connection to devices and substrates and ease of handling.
Flexible optical waveguides include polymer film optical waveguides described in, for example, Patent Document 3. Polymer films are formed as follows. A solution of a polymer or the like is applied on a substrate such as silicon by spin coating and is baked to form a lower cladding layer. In the same manner, a core layer is formed and then a mask pattern is formed with, for example, a Si-containing photoresist and dry-etched to form a core pattern. After that, an upper cladding layer is formed in the same manner as that in which the lower cladding layer is formed. Finally, the resultant optical waveguide is peeled from the substrate to fabricate a film-made optical waveguide. In particular, to make it easy to peel the optical waveguide, there is shown a method in which a thermally oxidized silicon substrate is used as the substrate and after the formation of the optical waveguide, the substrate having the optical waveguide thereon is immersed in hydrofluoric acid to separate the optical waveguide.
However, in the case of the above-mentioned film optical waveguide, each of the lower cladding, core, and upper cladding layers is formed by spin coating and baking. This method takes much time for forming each layer and in addition has problems arising from the fact that the form of the material is liquid. That is, since optical waveguides are fabricated by a film forming method that involves applying a liquid material on a substrate, management of film thickness is cumbersome and in addition, the resin applied on the substrate is still liquid before curing, so the resin will flow on the substrate to make it difficult to maintain uniformity of film thickness. Also, the method is not suitable for mass production of optical waveguides having a size of 10 cm or more because of use of silicon for substrates.
Further, the above-mentioned production method involves a step of dry etching, which is a high vacuum process, so dry etching must be performed for a very long period of time to fabricate multi-mode optical waveguides having a thick core layer.
Patent Document 1: Japanese Patent No. 3085666
Patent Document 2: Japanese Patent Application Laid-Open No. 6-228274
Patent Document 3: Japanese Patent Application Laid-Open No. 7-239422
Non-patent Document 1: Journal of Japan Institute of Electronics Packaging, Vol. 7, No. 3, pp. 213-218, 2004
Non-patent Document 2: IEEE Journal of Lightwave Technology, Vol. 16, pp. 1049-1055, 1998
Non-patent Document 3: Optics (“Kogaku”), vol. 3, No. 2, pp. 81-83, 2002